This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The molecular basis for alloreactive T cells and their specificity/degeneracy for pMHC ligands remain unresolved. Using a large scale screening approach involving 60 novel I-Ek alloreactive T cells and 83 naturally processed self-peptides, we identified nine T cells for which there were matching stimulatory self-peptides. Six of these T cells recognized single peptides with the same specificity observed for conventional recognition. Three of the T cells were stimulated by two or three distinct peptides, which had no sequence homology. These multiply reactive T cells recognized each pMHC specifically, but utilized distinct constellations of I-Ek contact residues. These studies reveal that allorecognition is highly specific, and that each T cell has an inherent capacity to recognize multiple distinct ligands, providing an explanation for the high frequency of alloreactivity.